In U.S. Pat. No. 4,109,635 to Rossborough of Aug. 29, 1978 a machine for cutting a sealing groove around the spigot end of a manhole riser is disclosed in which the machine is supported on the spigot end and centered on the longitudinal axis of the riser so that the groove is cut at a uniform radial distance from the longitudinal axis.
This patent is owned by the assignee of this application and the assignee has found that while such an axis centered groove is useful, it does not take into consideration the fact that there are no reinforcing rods in the cast spigot and it sometimes is not a true truncated cone due to depressions, slight collapse, bumps or the like that develops before, or during, the curing process.
Therefore, in the co-pending patent application of William P. Gundy, Ser. No. 605,463, filed Apr. 30, 1984 and entitled "Spigot Grinder and Groover", also owned by said assignee, a centering system is disclosed having two pairs of diametrically opposed sensors, which contact the outer face of the spigot, one in each quadrant to center the cutting and grinding apparatus on the end of the spigot. Each pair of sensors is oppositely disposed at 180.degree. from the other pair and each sensor is advanced and retracted by one of four hydraulic pistons and cylinders, each pair of hydraulic pistons and cylinders being controlled by one of a pair of master cylinders which cause the slave cylinders to move an equal amount.
The assignee's experience has been that this is an excellent system but it is costly, and requires dependence on electrically controlled hydraulic circuits. Thus, there is a need for a lower cost centering system to satisfy the demands of the trade.